Harnessing the bandwidth of optics for transmission of information and for computing is very much at the forefront of current reasearch and development efforts. This includes work in the area of "free space" optics, where the three-dimensional space (formerly referred to as "ether") is the communication medium between light emitting devices and light detecting devices. One advantageous characteristic of "free space" is that light beams can be intersected without comingling. Another advantageous characteristic is that a large number of beams can be handled in parallel, as a group, with a single optical setup. Still another advantage is that "free space" is indeed free; it does not need to be manufactured, and it costs nothing.
On the other hand, optics imposes its own constraints on the architecture of the systems that are designed. These constraints have been overcome to some extent, as exemplified by systems described, for example, in U.S. patent application Ser. No. 071,105, filed Jul. 8, 1987, and titled "Computational Origami", and U.S. patent application Ser. No. 219,623 filed Jul. 15, 1988 and titled "Optical Crossover Network". These and other free space systems have one thing in common, and that is the use of plane arrays of optical devices, and corresponding arrays of light beams. Typically also, a number of different beam arrays are required because a usable logic device will, in general, need at least two logical inputs. Depending on the type of device, it may also require one or more optical bias beams. This is akin to a transistor logic gate, where one employs a number of logic signals and a power supply source for operating the logic gate. Hence, there is a need in the field of free space optical information handling to operate with a plurality of beams and, in particular, there is a need to arrange for multiple arrays of beams, with each array being derived possibly from a different source or sources, to be incident on a desired array of optical devices. In other words, there is a need to combine beams and to separate beams.
On approach for combining or separating two beam arrays is to apply them to a beam splitter as shown, for example, in FIG. 1. Beam 11 is applied to cube beam splitter 10 at one face of the beam splitter, where it is split into beams 13 and 14. Beam 12 is applied to beam splitter 10 at another face of the splitter (orthogonal to the first face), wherein it is split into beams 15 and 16. Beams 14 and 15 exit beam splitter 10 at the same face (to the right) and thus they are combined. Alas, using simple beam splitters to achieve beam combining entails loss, since energy is diverted to beams 13 and 16.
Polarization-dependent beamsplitters can also be used to combine two beams, and such combining is achieved essentially but without loss. In the arrangement of FIG. 1 where the beam splitter is sensitive to the polarization mode of the incoming light, it can be arranged for the light of beam 11 to be so polarized that it passes through the beam splitter without deflection, thus placing no energy in beam 13. Similarly, it can be arranged for the light of beam 12 to be so polarized that it is deflected in the beam splitter, thus placing no energy in beam 16. The resultant beam combining that occurs within beam splitter 10 is lossless, but the combined beam is partially polarized in one mode and partially polarized in another mode.
Dichroic beamsplitters can be used, in principle, to combine beams of different wavelengths without loss. In practice, however, we may not wish to be constrained to use different wavelengths for the different beams, and devices may not work if such different wavelengths are used.
In a different environment, and for a different purpose, image combining has been accomplished with the use of a reflective grating. This approach is described in "Real Time Incoherent Optical-Electronic Image Subtraction," Dashiell et al., Optics Communications, Jun., 1973, pps. 105-108. The described approach passes one image through a reflective grating and reflects another image through the same grating. The grating in effect samples both images. The combined sample images are then applied to an image plane where the sampled combined image is converted to electronic signals and processed. This is akin to sampling a signal at a high rate (above Nyquist rate), combining the sampled signals, and filtering with an appropriate bandpass filter. Of course, there is loss associated with this approach because a portion of each image is missing. In fact, in its operation (vis-a-vis loss) the Dashiell et al. arrangement is identical the arrangement of FIG. 1, but optically more complex.